Commonly, semiconductor wafers are aligned prior to operations such as wafer probing, wafer stepping, wafer reading, wafer processing, etc. To achieve alignment of the wafer, two points on the wafer must be found; the location of the wafer center and the location of a wafer orientation feature, such as a notch or a flat cut into the typically round perimeter of the wafer, as shown in FIGS. 1A and 1B.
Opto-mechanical methods are known for finding an orientation feature cut into the perimeter of a wafer. For example, it is known to direct light from a laser or LED array at a point on the perimeter of a wafer. The wafer is then rotated, while repeatedly measuring the radial position of the wafer edge using the directed light. After rotating through about one revolution, there is sufficient information regarding the radial position of the edge that at least an approximate position of the center of the wafer can be ascertained using a known circle-fitting equation.
Once a circle is fit to the radial position information, points that significantly deviate from the circle are analyzed to localize the flat (or notch) of the wafer. With the wafer center and flat (or notch) locations known, the wafer handling equipment can then place the wafer accurately for execution of the next operation.
Electro-mechanical devices are also known for determining the orientation of a wafer, such as the apparatus disclosed in U.S. Pat. No. 4,457,664.
U.S. patent application Ser. No. 08/503,574, filed Jul. 18, 1995, entitled SYSTEM FOR FINDING THE ORIENTATION OF A WAFER, assigned to Cognex Corporation, teaches a machine vision method for automatically determining the location of center of a wafer and the flat (or notch) of the wafer. Use of this method dramatically reduces contact with and handling of the wafer necessary to locate the center and orientation feature of a wafer, while also significantly reducing the time required to perform these operations. By reducing the amount of wafer handling, the likelihood of contamination of the wafer is reduced. By decreasing the time needed to analyze information regarding the coordinates of points along the circumference of the wafer, parts can move more efficiently from place to place in the wafer fabrication facility, thereby increasing the throughput of the fabrication facility.
FIG. 2 shows a video camera 24 with standard optics 32 for capturing a wafer image to be processed by a machine vision system (not shown). Here, the wafer 30 is viewed by the camera 24 directly, the camera 24 and optics 32 alone serving as an image formation system. The focal length of the optics 32 determines the camera working distance d.sub.W 34, i.e., the direct distance from the optics 32 of the video camera 24 to the surface of the wafer 30.
Effective camera height EC.sub.H is defined as the minimum enclosing distance 35 between two hypothetical parallel plates 36 and 38, each plate also being parallel to the surface of the wafer 30. In FIG. 2, for example, where the camera is oriented perpendicularly to the wafer surface, the effective camera height 35 is simply the length of the camera (including the lens 32). If the camera were oriented 45 degrees with respect to the wafer surface, for example, the effective camera height EC.sub.H would be the distance between the hypothetical plates 36 and 38, where the hypothetical plate 36 just touches the top corner of the camera 24 that is farthest from the wafer 30, and the hypothetical plate 38 just touches the bottom corner of the camera 24 (including the lens 32) that is nearest to the wafer 30.
The effective profile height EP.sub.H 40 of an image formation system is herein defined as the minimum enclosing distance between the surface of the wafer 30 and a hypothetical parallel plate 42 that encloses the farthest element of the image formation system. In FIG. 2, for example, where the image formation system consists solely of a camera 24 and its lens 32, and the camera 24 is oriented parallel to the direction of the working distance, the effective profile height EP.sub.H 40 is the distance of the hypothetical enclosing plate 42 that just touches a farthest point on the camera 24 from the surface of the wafer 30. Also, in this particular case, the effective profile height EP.sub.H 40 is also equal to the sum of the effective camera height 35 and the working distance 34.
Using a lens with an 8.5 mm focal length provides the shortest working distance that results in acceptable optical distortion. A shorter focal length would decrease the working distance, and therefore would decrease the effective profile height, but would also result in unacceptable image distortion.
However, even a camera positioned at this shortest working distance from the wafer, having acceptable image distortion, results in other problems. These problems include an inability to operate within small or narrow enclosures of some existing wafer fabrication equipment, due to an effective profile height EP.sub.H that cannot be accommodated within the enclosures of some existing wafer fabrication equipment. These problems also include difficulties achieving stability against motion of the camera with respect to the object under study when the camera is supported by an extended camera mounting arm. For example, when a mounting arm supports the camera at a distance range of between 20 inches and 25 inches from the wafer, the field of view (FOV) of the camera includes the entire wafer, but the camera may not be sufficiently stable against even small vibrations that can result in motion-induced image distortion.